Electrical resistor



Dec. 3, J. BARRINGTQN 3,414,864

ELECTRICAL RESISTOR Filed June 8, 1967 l/V VE N 70/? JONA THAN 8A RR/NG TON gw a ATTORNEY United States Patent Office Patented Dec. 3, 1968 ABSTRACT OF THE DISCLOSURE An electrical resistor comprising a cylindrical substrate of an electrical insulating material having one or more longitudinally extending channels in its cylindrical surface. A film of an electrical resistance material is coated on the cylindrical surface of the substrate and the surface of the channel or channels. Terminals are connected to the ends of the substrate and the resistance material film. The resistance value of the resistor can be adjusted to a desired resistance value by small increments by removing narrow bands of the resistance material film from around the cylindrical surface of the substrate between the edges of the channel or channels leaving the surface of the channel or channels coated with the resistance material film. The narrow bands of the resistance film can be removed by either cutting a helically extending groove through the resistance film on the cylindrical surface of the substrate so as to electrically isolate the bands, or by completely removing the resistance material, such as by grinding, to provide a gap in the resistance material film on the cylindrical surface of the substrate.

BACKGROUND In the manufacture of discrete electrical resistors of the type comprising a cylindrical substrate of an electrical insulating material having a film of an electrical resistance material coated on the surface thereof, one problem is obtaining resistors of a desired resistance value. Since the resistance value of such a resistor is a function of the width and length of the path of the resistance material film, it has been the practice to obtain a desired resistance value by adjusting the width and length of the resistance material path.

One method generally used for adjusting the resistance value of a film type resistor by varying the width and length of the resistance material path, a method commonly called spiralling, comprises cutting a fine groove through the resistance film which groove extends helically around the substrate. This cuts the resistance material film into a narrow, long path which extends helically around the substrate so as to increase the resistance value of the resistor. The width and length of the helical resistance material path depends on the pitch of the helical groove and the number of turns of the groove. By measuring the resistance value of the resistor as the groove is being cut, the spiralling can be stopped when the desired resistance value is reached. Although this method of adjusting the resistance value of a resistor permits the achievement of very accurate resistance values, it has the disadvantage that it is difficult to achieve only very small increments of adjustment particularly when adjusting resistors having very low resistance values.

Another method heretofore used for adjusting the resistance value of a film type resistor is a lateral adjust method. This method of adjustment comprises removing a narrow strip of the resistance material longitudinally along the substrate. This can be achieved by passing a narrow width grinding wheel longitudinally along the surface of the coated substrate. By forming two or more of such strips at circumferentially spaced points around the substrate, there is provided two or more parallel resistance paths having a higher resistance value than the original resistance value of the resistor. The width, length and number of the resistance paths determine the final resistance value of the resistor. By measuring the resistance value of the resistor during the lateral adjust, the removal of the resistance material film can be stopped when the desired resistance value is reached. This type of adjustment of the resistance value of the resistor also has the disadvantage that it is difficult to achieve only small increments of adjustment of the resistance value.

SUMMARY It is an object of the present invention to provide a novel construction of a film type electrical resistor.

It is another object of the present invention to provide a novel construction of a film type electrical resistor which can be easily adjusted as to its resistance value by small increments of change.

It is a further object of the present invention to provide a construction of a film type electrical resistor which can provide a resistor of low resistance value and can be adjusted as to its resistance value by small increments.

It is a still further object of the present invention to provide a film type electrical resistor which includes a cylindrical substrate having one or more longitudinally extending channels in its outer surface with the resistance material film being coated on the surface of the channel or channels as well as on the cylindrical surface of the substrate so that the resistance value of the resistor can be adjusted by small increments by removing narrow substantially cylindrical bands of the resistance material film from the cylindrical surface of the substrate between the edges of the channel or channels.

Other objects will appear hereinafter.

Brief description of drawings For the purpose of illustrating the invention, there is shown in the drawings forms which are presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIGURE 1 is a plan view of the electrical resistor of the present invention, showing one method of adjusting the resistance value of the resistor.

FIGURE 2 is a sectional view taken along line 22 of FIGURE 1.

FIGURE 3 is a sectional view taken along line 33 of FIGURE 1.

FIGURE 4 is a plan view of the electrical resistor of the present invention showing another method of adjusting the resistance value of the resistor.

FIGURE 5 is a sectional view taken along line 55 of FIGURE 4.

FIGURE 6 is a sectional view taken along line 66 of FIGURE 4.

Description of invention Referring initially to FIGURES l-3, the electrical resistor of the present invention is generally designated as 10. Resistor 10 comprises a cylindrical substrate 12 of an electrical insulating material, such as a plastic, glass or ceramic. Substrate 10 has three channels 14 in its cylindrical surface. The channels 14 extend longitudinally along the substrate 10 from end to end thereof and are uniformly spaced around the cylindrical surface of the substrate. Although the resistor 10 is shown as having three channels 14 in the substrate 12, as will be explained, the substrate can have any desired number of the channels. Coated on the cylindrical surface of the substrate 10 and the surfaces of the channels 14 is a film 16 of an electrical resistance material. The resistance material of the film 16 may be any well-known resistance material such as carbon or a ;metal either alone or in a bonder such as a plastic, ceramic or glass. Metal terminal caps 18 fit over the ends of the substrate 12 and contact the resistance film 16. Terminal wires 20 are secured to and extend longitudinally from the terminal caps 18.

To adjust the resistance value of the resistor 10, a narrow groove 22 is cut through the resistance material film 16 to the substrate 12. The groove 22 extends helically around the substrate 12. Although the groove 22 may extend slightly into the surface of the substrate 12, it is not as deep as the longitudinal channels 14 in the substrate. Thus, the helical groove 22 is interrupted by the channels 14, and the groove 22 does not cut into the portion of the resistance film covering the surface of the channels 14. As the groove 22 is extended along the substrate 12, narrow bands of the resistance material film which extends around the cylindrical surface of the substrate between the channels 14 are electrically isolated so that along the spiralled portion of the resistor 10, the active path of the resistance material is only along the channels 14. Thus, the spiralling of the resistor does not change the length of the active path of the resistance material film 16, which is always the distance between the terminal caps 18, but does decrease the circumferential width of that portion of the length which is spiralled so as to increase the resistance value of the resistor. Therefore, as the groove 22 is extended along the substrate 12, the resistance value of the resistor 10 progressively increases. However, since each band of the resistance material film 16 which is isolated is narrow, each turn of the helical groove 22 causes only a small increment of change of the over-all resistance value of the resistor 10.

The number of channels 14 in the substrate 12 deter mines the magnitude of each increment of change in the resistance value with the magnitude of the increment decreasing with the increasing of the number of the channels. For example, Table I shows the results of spiralling a group of film type resistors of the same size but having different number of channels. As can be seen from Table I, spiralling resistor 1, which is of the type heretofore known without any channels, with 11 /2 turns caused the resistance value to increase by a magnification factor of 324. However, spiralling resistors 3 and 5, which are of the construction of the present invention with 1 and 3 channels respectively, with a similar number of turns only caused the resistance value to increase by a magnification factor of 5.60 and 1.54 respectively. Thus, it can be seen that the resistor 10 of the present invention can be adjusted as to its resistance value by small increments so that even resistors of low resistance value can be easily adjusted.

Referring to FIGURES 4-6, a modification of the resistor of the present invention is generally designated as 10 Resistor 10 like resistor 10 of FIGURES 1-3, comprises a cylindrical substrate 12 of an electrical insulating material having longitudinally extending channels 14 in its outer surface. Although the substrate 12 is shown as having three channels 14 it can have any desired number of the channels. A film 16 of an electrical resistance material is coated on the cylindrical surface of the substrate 12 and the surface of the channels 14 Metal terminal caps 18 are mounted on the ends of the substrate 12 and contact the resistance film 16 Metal terminal wires 20 are secured to and extend longitudinally from the terminal caps 18 To adjust the resistance value of the resistor 10 a band of the resistance material film 16 is removed from around the cylindrical surface of the substrate 12 providing a gap 22 in the portion of the resistance material film on the cylindrical surface of the substrate. Thus, along the portion of the substrate where the gap 22 exits, the active path of the resistance film 16 is only along the surface of the channels 14 This increases the resistance value of the resistor 10 since the width of a portion of the resistive path of the resistance film 16 is reduced. By widening the gap 22 the resistance value of the resistor 10 is progressively increased. This method of adjusting the resistor of the present invention is similar to the spiralling method shown in FIGURES 1-3 except that in the spiralling method the narrow bands of the resistance material film are only electrically isolated from the rest of the resistance material film whereas in this method the narrow bands of the resistance material film are completely removed. However, the electrical result is the same and the method of FIGURES 4-6 provides small increments of adjustment of the resistance value.

Table II shows the results of adjusting the resistance value of a resistor 10 of the present invention. The resistor had three channels 14 and an active resistance path length of 0.625 inch. The resistor was rotated about its own longitudinal axis and a rotating diamond grinding wheel was placed against the cylindrical surface of the resistor and moved longitudinally along the resistor to progressively remove the resistance material film from the cylindrical surface of the substrate. The gap was initiated 0.030 inch from one of the terminal caps and the resistance value of the resistor was measured at various steps as the gap was widened. It can be seen from Table II that, although the resistance value of the resistor increased as the gap width increased, the incremental change was small and, even when the gap width equalled almost of the total active length of the resistance film, the magnification factor was still less than 2.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

I claim:

1. An electrical resistor comprising a cylindrical substrate of an electrical insulating material, said substrate having a substantially straight channel in its cylindrical surface extending substantially parallel to the longitudinal axis of the substrate, a film of an electrical resistance material coated on the cylindrical surface of the substrate and the surface of the channel, and terminals secured to the ends of the substrate and contacting said resistance material film.

2. An electrical resistor in accordance with claim 1 in which at least one band of the resistance material film around the cylindrical surface of the substrate between the edges of the channel is removed from the active path of the resistance material film so that a portion of the active path of the resistance material film is only along the surface of the channel.

3. An electrical resistor in accordance with claim 2 including a narrow groove through the resistance material film and extending helically around the substrate said helical groove removing a band of the resistance material film on the cylindrical surface of the substrate from the active path of the resistance material film.

4. An electrical resistor in accordance with claim 2 in which a portion of the resistance material film is completely removed from around the cylindrical surface of the substrate so as to provide a gap in the resistance material film along the cylindrical surface of the substrate.

5. An electrical resistor in accordance with claim 1 in which the substrate has a plurality of longitudinally extending channels in its cylindrical surface and the resistance material film is coated on the surface of all the channels.

6. An electrical resistor in accordance with claim 5 in which bands of the resistance material film around the cylindrical surface of the substrate between the edgesof the channels are removed from the active path of the resistance material film so that a portion of the active path of the resistance material film is only along the surfaces of the channels.

7. An electrical resistor in accordance with claim 6 ineluding a narrow groove through the resistance material film and extending helically around the substrate, said helical groove removing a band of the resistance material film on the cylindrical surface of the substrate from the active path of the resistance material film.

8. An electrical resistor in accordance with claim 6 in which a portion of the resistance material film is completely removed from around the cylindrical surface of the substrate so as to provide a gap in the resistance material film along the cylindrical surface of the substrate.

References Cited UNITED STATES PATENTS 1,767,715 6/1930 Stoekle 338-300 LEWIS H. MYERS, Primary Examiner.

ELLIOTT A. GOLDBERG, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION atent No. 3,414,864 December 3, 1968 Jonathan Barrington It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, line 4, 'IRC, Inc., Philadelphia, Pa." should read TRC Inc., corporation of Ohio Signed and sealed this 10th day of March 1970.

SEAL) tttest:

dward M. Fletcher, Jr.

tttesting Officer Commissioner of Patents 

